1. Field of the Invention
The invention relates to clutches and, more particularly, relates to synchronized clutches effecting sequential nonpositive and positive connection of a driven element to a drive element.
2. Discussion of the Related Art
Synchronized clutches are known and typically include a nonpositive coupling such as a system of friction discs which initially and nonpositively couple a driven element to a drive element to accelerate the driven element, and a positive coupling such as a pair of mating rings which subsequently positively couple the driven element to the drive element.
One such clutch is disclosed in U.S. Pat. No. 1,681,714 to Tullar. The clutch disclosed by the Tullar patent employs as the nonpositive coupling a plurality of friction discs which serve to accelerate the driven element, and employ as the positive coupling inclined teeth which subsequently lockingly engage a surface element coupled to the flywheel comprising the drive element. Engagement of both the nonpositive coupling and positive coupling of the clutch is controlled by a spring which translates both couplings longitudinally along the shaft forming the driven element upon release of a lever.
The clutch disclosed in the Tullar patent, while being at least to some extent synchronized, exhibits several disadvantages. For instance, operation of both the nonpositive and positive couplings of the clutch is controlled by the same spring, which exerts forces on the clutch which are independent of the rotational speed of the driven shaft. The nonpositive and positive couplings are both parts of the same operation and occur very abruptly because both are initiated when the clutch lever releases the main spring. There is, as a result, no control over synchronization or positive engagement. Thus, positive engagement will always take place regardless of the rotational speed of the drive element, will always occur at a time determined by the constant net actuating force imposed by the main spring, and can be terminated only by manual actuation of a lever to compress the main spring.
Another synchronized clutch, disclosed in U.S. Pat. No. 3,161,270 to Aschauer, also provides sequential nonpositive and positive engagement of a driven shaft to a drive device including a flywheel. However, the friction discs forming the nonpositive coupling of the clutch and the engaging ring comprising the positive coupling are engaged hydraulically rather than by operation of a spring. The pressurized hydraulic fluid can be applied to the nonpositive coupling and positive coupling under independent controls. Thus, positive coupling need not always occur subsequent to nonpositive coupling.
The clutch disclosed by Aschauer, though in some respects arguably superior to that disclosed by Tullar, exhibits drawbacks and disadvantages. For instance Aschauer's clutch, like Tullar's, engages its friction or nonpositive coupling at a rate which is predetermined. That is, the force imposed on the nonpositive friction coupling is constant and is determined by the effective surface area of the piston elements and upon the pressure of the hydraulic fluid used to actuate the clutch. Moreover, engagement of the positive coupling is independent of the rotational speed of the driven element. Also, as in Tullar, disengagement of the positive coupling does not occur automatically upon deceleration of the driven element, but instead requires removal of the application forces through exhaust of pressurized hydraulic fluid from the associated chamber.
Both Tullar and Aschauer also exhibit the drawbacks of being relatively bulky and thus of being poorly suited for use in applications in which weight and/or compactness are of concern. Both devices are also relatively complex, thus rendering them less than ideally suited for applications which require maintenance-free operation for extended periods of time.